1. Field of the Invention
The invention relates to a molten carbonate fuel cell and a power generation system including the molten carbonate fuel cell.
2. Description of the Related Art
A molten carbonate fuel cell has many advantages in comparison with conventional power generation systems. For instance, it provides higher efficiency and less pollution to environment. Thus, the molten carbonate fuel cell has been studied and developed in many countries all over the world as a next power generation system following hydraulic, thermal and nuclear power generation.
FIG. 1 illustrates one of conventional MCFC power generation systems consuming natural gas as fuel. The illustrated power generation system includes a reformer 10 for reforming fuel gas 1 such as natural gas into anode gas 2 containing hydrogen gas therein, and a molten carbonate fuel cell 12 for producing electric power from the anode gas 2 and cathode gas 3 containing oxygen and CO.sub.2. The anode gas 2 produced in the reformer 10 is supplied to the fuel cell 12, and consumed the most part, for instance, about 80% for power generation reaction of MCFC to thereby transform into anode exhaust gas 4, which is supplied to a combustion chamber of the reformer 10. In the reformer 10, combustible ingredients in the anode exhaust gas 4 such as hydrogen, carbon monoxide and methane are burned to produce high temperature combustion gas by which a reforming tube is heated to thereby reform fuel passing through the reforming tube. Combustion exhaust gas 5 leaving the reformer 10 joins pressurized air 6 supplied from a pressure recovery device 15 to thereby produce cathode gas 3, which supplies requisite carbon dioxide and oxygen to a cathode section C of the fuel cell 12. The cathode gas 3 makes partial reaction for power generation in the fuel cell 12 to thereby produce cathode exhaust gas 7, a part of which is circulated to the upstream of the fuel cell 12 by means of a cathode recirculation blower 14 and remainder of which is pressure-recovered by means of a pressure recovery apparatus 15 including a turbine 16 and an air-compressor 17 and also heat-recovered by means, of a heat recovery apparatus 18, and then exhausted to atmosphere. Before entering the reformer 10, the fuel gas 1 is mixed with steam 8.
FIGS. 2A and 2B schematically illustrate mechanical structure of the fuel cell 12. As illustrated, the molten carbonate fuel cell 12 includes electrolyte plate t, a set of anode, a and cathode c each disposed on one of opposite surface of each of the electrolyte plate t, and separators s for mechanically separating the anode and cathode gas. The electrolyte plate t is a planar plate made of ceramic powder, and holds carbonate in gaps therein with the carbonate being molten at high temperature. The anode a and cathode c both made of sintered metal powder and formed in a planar plate sandwich the electrolyte plate t therebetween. The anode a, electrolyte plate t, and cathode c form a single cell. The molten carbonate fuel cell 12 is comprised of a plurality of cells forming a layer-stacked cell in which the cells are sandwiched between the separators s.
That is, a single cell is comprised of the planar electrolyte plate t sandwiched between the anode a and the cathode c, a current collector made of a punched plate and supporting, the electrolode plate for protection, and the separator s made of metal and disposed outside the current collector. In an internal manifold type fuel cell, the separators s form passages through which fuel gas and oxidant gas are supplied to anode and cathode. In a layer-stacked cell, the separators s makes possible to stack a plurality of cells and make them integrated with one another. It should be noted that various types of separator have been manufactured for obtaining proper function thereof. For instance, one of conventional separators is comprised of a flat plate combined with corrugate plates or pressed plate. FIG. 2A illustrates a separator comprised of pressed plates, whereas FIG. 2B illustrates a separator comprised of a flat plate combined with corrugate plates.
The above, mentioned conventional molten carbonate fuel cell has problems as follows.
First, a separator in a conventional molten carbonate fuel cell is required to have high accuracy, high electrical conductivity and high corrosion resistance in order to have many functions of: forming a passage through which fuel gas and oxidant gas is supplied to an anode and a cathode; preventing those two gases from mixing with each other; having electrical conductivity; and forming wet seal w illustrated in FIG. 2B by making direct contact with a highly corrosive electrolyte plate at high temperature. As a result, a separator is complicated in structure and costs too much for fabrication.
Secondly, it is not possible to exchange one of stacked cells, even if it is inferior, since a lot of cells are integrally stacked to use as a layer-stacked cell. Electrolyte is decreased by corrosion and evaporation at 550.degree..about.700.degree. C. which results in shorter lifetime of a cell. However, it is quite difficult to supplement electrolyte to a cell because of the stacked structure.
In a power generation system including the conventional fuel cell, an overall structure thereof is too complicated and large in size, and thus the power generation system costs too much.